The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The invention relates to production of fluids from subterranean formations. More particularly, it relates to stimulation of flow through formations by hydraulic fracturing. Most particularly, it relates to methods of optimizing fracture conductivity by propping fractures in a formation stratum so that the proppant is distributed heterogeneously in the fracture, and in some embodiments, the fracture containing substantial voids with little or no proppant.
Hydraulic fracturing is a primary tool for improving well productivity by placing or extending highly conductive fractures from the wellbore into the reservoir. Conventional hydraulic fracturing treatments generally are pumped in several distinct stages. During the first stage, normally referred to as the pad, a fluid is injected through a wellbore into a subterranean formation at high rates and pressures. The fluid injection rate exceeds the filtration rate (also called the leakoff rate) into the formation, producing increasing hydraulic pressure. When the pressure exceeds a threshold value, the formation cracks and fractures. The hydraulic fracture initiates and starts to propagate into the formation as injection of fluid continues.
During the next stage, proppant is mixed into the fluid, which is then called the fracture fluid, frac fluid, or fracturing fluid, and transported throughout the hydraulic fracture as it continues to grow. The pad fluid and the fracture fluid may be the same or different. The proppant is deposited in the fracture over the designed length, and mechanically prevents the fracture from closure after injection stops and the pressure is reduced. After the treatment, and once the well is put on production, the reservoir fluids flow into the fracture and filter through the permeable proppant pack to the wellbore. The production of reservoir fluids depends upon a number of parameters, such as formation permeability, proppant pack permeability, hydraulic pressure in the formation, properties of the production fluid, the shape of the fracture, etc. One of the most essential parameters and one that can be designed, controlled and adjusted in hydraulic fracturing is the hydraulic conductivity of the fracture (the proppant pack permeability multiplied by the fracture width). There are numerous cases in which an increase in the hydraulic conductivity of a proppant pack above the limits of conventional technology would result in significant improvements in stimulation economics.
There have been prior attempts at heterogeneous proppant placement. Some prior inventions aim to increase the hydraulic conductivity of a fracture through the heterogeneous placement of proppants in a layer of a formation. Many of these inventions involve pumping different types of slurries or fluids in discrete intervals, known in the industry as “slugs” or “stages”. This is claimed to provide higher conductivity fractures than those obtained from conventional treatments, and to increase fracture conductivity by replacing the homogeneous proppant pack with a heterogeneous proppant pack. Proppant structures, sometimes referred to as pillars, clusters, or posts, are placed at intervals throughout the created fracture. These pillars have sufficient strength to hold the fracture partially open under closure stress. The space between pillars forms a network of interconnected open channels, available for flow. This results in a significant increase of the effective hydraulic conductivity of the overall fracture.
Patent application publications US20060113078A1 and US20060113080A1 describes methods of propping at least one fracture in a subterranean formation, by attempting to introduce a plurality of proppant aggregates into at least one fracture, forming a plurality of proppant aggregates, each of which includes a binding fluid and a filler material. In U.S. Pat. Nos. 3,850,247, 3,592,266, 5,411,091, 6,776,235 and patent application publication US20050274523, high conductivity channels are created by pumping alternating intervals of fracturing slurries which are different in at least one of their parameters. For example, in U.S. Pat. No. 3,592,266 it is proposed to create heterogeneity in a proppant pack by pumping alternating volumes of fluids that are significantly different in their viscosities. In U.S. Pat. No. 6,776,235 the fluids differ in their proppant carrying capacity and/or in the concentration of proppant. Each of the above mentioned references are incorporated herein, in their entirety, by reference thereto.
However, in these methods of heterogeneous proppant placement there may be limited control over the location of the pillars. In addition, there is a tendency for the pillars to be very long and to extend the entire height of the fracture (assuming a vertical fracture) and so the channels between the pillars do not lead to the wellbore, and so cannot provide superior pathways from the formation all the way to the wellbore.
A method of heterogeneous proppant placement in which there is better control over the location of the pillars would be of benefit. In addition, placement such that the pillars do not extend the entire height of the fracture (assuming a vertical fracture) but are themselves interrupted by channels so that the channels between the pillars form pathways that do lead to the wellbore, would be very beneficial. It is one goal to provide such heterogeneous proppant placement.